Your problem may be two fold:
1) You aren't using PURE SEB, and
2) you aren't boiling it long enough
it's possible that you aren't using the correct type of pay to cook it in either. Here are the directions on how to do it from felinecrf.org:
Sprinkle 1 to 1.5 tsp of slippery elm bark powder onto one cup of cold water. If you are using slippery elm bark from capsules, you need pure slippery elm bark with no fillers, otherwise the syrup will usually not thicken properly.
Let it sit until the powder is damp, then bring to the boil stirring constantly. Use a stainless steel or heat-proof glass pan, not an aluminium or non-stick one. Reduce the heat and simmer, still stirring constantly, until it thickens, about 3 minutes or so. It will be the consistency of watery egg whites.
From that same site, here is a recipe for making it in the microwave, although I've found that a 2-cup measuring cup isn't quite big enough, since I kept having to open my microwave up to keep it from boiling over .
For the non-cooks among us, one of my support group members came up with this method.
Take a glass container which holds two cups. Put one cup of water in the cup. Add 1 to 1.5 tsp of slippery elm bark powder.
Heat in the microwave on high for about 1.5 minutes. Remove from the microwave and stir, then put it back in for about another 30 seconds. Keep an eye on it because you do not want it to boil over. Remove from the microwave and stir again.